1. Field of the Invention
The present invention relates to a beam homogenizer to homogenize energy distribution of a beam spot on an irradiated surface in a particular region. Moreover, the present invention also relates to a laser irradiation apparatus using the beam homogenizer. Furthermore, the present invention relates to a method for manufacturing a semiconductor device using the laser irradiation apparatus.
2. Related Art
In recent years, a technique has been extensively researched in which the laser annealing is performed to a non-single crystal semiconductor film (the non-single crystal semiconductor includes an amorphous semiconductor and a semiconductor having crystallinity such as poly-crystal or micro-crystal, which is not single crystal) formed over an insulating substrate such as a glass substrate. It is noted that the laser annealing described herein indicates a technique to recrystallize an amorphous layer or a damaged layer formed in a semiconductor substrate or in the semiconductor film and a technique to crystallize the non-single crystal semiconductor film formed over the substrate. Moreover, a technique applied to planarize or modify a surface of the semiconductor substrate or the semiconductor film is included in the laser annealing.
The laser annealing is performed in the crystallization because the glass substrate has a low melting point. The laser can give high energy only to the non-single crystal semiconductor film without changing the temperature of the substrate that much.
It is preferable to perform the laser annealing in such way that a pulsed laser beam having high output such as an excimer laser is shaped into a square spot having a length of several cm on a side or into a rectangular spot having a length of 10 cm or more on a longer side at an irradiated surface through an optical system and that an irradiation position of the beam spot is scanned relative to the irradiated surface because this method can enhance productivity and is superior industrially. Among the rectangular beam spots, a rectangular beam spot having a high aspect ratio is referred to as a linear beam spot in this specification.
In particular, unlike a punctate beam spot requiring to be scanned from front to back and from side to side, the linear beam spot can provide high productivity because the linear beam spot can be irradiated to the large irradiated surface by scanning the linear beam spot only in a direction perpendicular to the long-side direction of the linear beam spot. The laser beam is scanned in the direction perpendicular to the long-side direction of the linear beam spot because it is the most effective way to scan the laser beam. Because of such high productivity, at present, the laser annealing process mainly employs the linear beam spot obtained by shaping the beam spot emitted from a pulsed excimer laser through an appropriate optical system.
FIGS. 12A and 12B show an example of an optical system to transform the sectional shape of the beam spot into linear on the irradiated surface. The optical system shown in FIGS. 12A and 12B is an extremely general optical system. This optical system not only transforms the sectional shape of the beam spot into linear, but also homogenizes the energy distribution of the beam spot on the irradiated surface at the same time. Generally, the optical system for homogenizing the energy distribution of the beam spot is referred to as a beam homogenizer. The optical system shown in FIGS. 12A and 12B is also the beam homogenizer.
When a XeCl excimer laser (having a wavelength 308 nm) is used as a light source, the above optical system is made of quartz. When a laser having a shorter wavelength is used as the light source, the optical system is made of fluorite, MgF2, or the like.
First, a side view of FIG. 12A is explained. A laser beam emitted from a XeCl excimer laser oscillator 1201 is divided in one direction through cylindrical lens arrays 1202a and 1202b. This direction is herein referred to as a vertical direction. When a mirror is inserted in the optical system, the vertical direction is bent to the direction of the laser beam bent by the mirror. In this structure, the laser beam is divided into four beams. These divided beams are combined into one beam spot once by a cylindrical lens 1204. The beam spots separated again are reflected by a mirror 1206 and then are condensed into one beam spot again on an irradiated surface 1208 by a doublet cylindrical lens 1207. The doublet cylindrical lens is a lens consisting of two cylindrical lenses. This homogenizes the energy distribution of the linear beam spot in the vertical direction and determines the length thereof in the vertical direction.
Next, a top view of FIG. 12B is explained. The laser beam emitted from the laser oscillator 1201 is divided in a direction perpendicular to the vertical direction by a cylindrical lens array 1203. The direction perpendicular to the vertical direction is herein referred to as a horizontal direction. When a mirror is inserted in the optical system, the horizontal direction is bent to the direction of the beam bent by the mirror. In this structure, the laser beam is divided into seven beams. These divided beams are combined into one beam spot by a cylindrical lens 1205 on the irradiated surface 1208. A dotted line shows a correct optical path and correct positions of the lens and the irradiated surface in the case not disposing the mirror 1206. This homogenizes the energy distribution of the linear beam spot in the horizontal direction and determines the length thereof in the horizontal direction.
As described above, the cylindrical lens arrays 1202a, 1202b, and 1203 are the lenses to divide the beam spot of the laser beam. The homogeneity of the energy distribution of the obtained linear beam spot depends on the number of the divided beam spots.
In general, the excimer laser emits a rectangular laser beam having an aspect ratio in the range of approximately 1 to 5. The beam spot of the laser beam has Gaussian distribution where the intensity is higher toward the center. The optical system shown in FIGS. 12A and 12B transforms the beam spot so as to form the beam spot having homogeneous energy distribution and having a size of 320 mm×0.4 mm
The linear beam spot shaped by the above structure is irradiated as being overlapped in such a way that the linear beam spot is displaced gradually in the direction of the short side of the linear beam spot. Such an irradiation method makes it possible to perform the laser annealing to the whole surface of the non-single crystal silicon film to crystallize it or to enhance its crystallinity. In a mass-production factory, at present, the laser annealing is performed to the semiconductor film using the linear beam spot shaped by the optical system as above.
Some beam homogenizers use a reflection mirror. (For example, patent document 1)    [Patent Document 1] Japanese Patent Unexamined Publication No. 2001-291681 bulletin
However, a laser irradiation apparatus using the pulsed excimer laser has a problem that, for example, the homogeneity of the energy distribution of the beam spot on the irradiated surface deteriorates because of the fluctuation of a beam axis, which is explained later, or the change in the divergence angle of a laser beam due to the change in the oscillation condition of the excimer laser or due to the cleaning of the window for isolating the gas, which is the laser medium of the excimer laser, from the outside. Therefore, such a laser irradiation apparatus is not yet of high quality for the mass production. The term “beam axis” herein used means a path in which the laser beam travels. The fluctuation or the change of the beam axis means that of the travel direction of the laser beam including the parallel shift of the travel direction of the laser beam.